The invention generally relates to the field of heterogeneous structures. In particular, the invention pertains to methods of forming an assembly that includes a layer of a first material having a first coefficient of thermal expansion on a second substrate of a second material having a different second coefficient of thermal expansion. These methods are useful in forming semiconductor assemblies.
A known structure having two substrates with different coefficients of thermal expansion is shown in FIG. 1. The structure includes a substrate 12 with a first coefficient of thermal expansion and a film or layer 15 having a second coefficient of thermal expansion. A quartz-on-silicon system, which can be used in optical applications such as in the production of displays, is one example of such a structure. Silicon has a coefficient of thermal expansion of 3.59×10−6/C., and that of quartz is 6×10−7/C. Other examples are silicon-on-sapphire, silicon-on-glass, silicon carbide-on-glass, germanium-on-glass, or germanium-on-silicon structures.
When two substrates are assembled with different coefficients of thermal expansion, for example, having differences of at least 10% or 20% at ambient temperature (20° C.), or during any subsequent treatment of two assembled substrates, temperature increase stages may take place, for example, in order to strengthen the bonding interface. When the thickness of the substrates is comparable, substantially identical or close to each other and when they behave as thin layers, variations in the behavior of one of the two surfaces with respect to the other can then result in at least one of the two substrates breaking due to a stress release phenomenon. This can occur when the temperature reaches a few hundred degrees (for example a temperature in the range of about 200° C. to about 600° C.).
In general, variations in temperature, for example in the range of about 200° C. to about 600° C., can cause stresses due to differences in the coefficient of thermal expansion. These temperature variations can also cause delamination or detachment of the substrates or layers that are present, and/or plastic deformation and/or fractures and/or breakage of one or more of the substrates or layers. Thus, it would be beneficial to have a method for providing such structures that avoid these problems during phases in which the temperature is changed.
A variety of techniques are known and currently used to produce structures such as those shown in FIG. 1.
A first technique shown in FIG. 2 uses ion implantation into a substrate 10 to form a weakened thin layer 13 which extends substantially parallel to the surface 16 of the substrate 10. The weakened thin layer defines a transfer layer 15 that will be transferred onto the second substrate. The thickness of each of the two substrates 10 and 12 are comparable or close (i.e., within ±10 to 15%). The substrates are then assembled with face 16 against face 18 using a wafer bonding (or direct contact) technique. The specific techniques used for bonding the substrates include adhesive bonding, molecular bonding, or anodic bonding.
Pre-annealing is then carried out at a given temperature and with a limited thermal budget that is lower than the budget that can produce thermal fracturing of the substrate 10. The thermal budget is the product of the duration of the heat treatment and the temperature of the heat treatment. The conditions employed therefore do not cause thermal fracturing of the substrate 10. The substrate 10 is then detached mechanically, for example by using a blade which provides the necessary additional energy.
Another technique involves mechanically and/or chemically thinning the substrate 10 after bonding the two substrates. Mechanical thinning is carried out by lapping and polishing. Chemical thinning involves the use of a substance such as TMAH (tetramethylammonium hydroxide). These two techniques do not allow the portion 14 that is removed from the substrate 10 to be recovered and re-used.
In a further technique, an anodic deposit can be formed on a substrate, for example a glass substrate, which is used as an anode. In this case, bonding occurs at a given temperature which permits the transfer process to be activated at temperatures lower than those required for molecular bonding. Such operation eliminates thermal stress in the layers.
The three techniques described above suffer from some major disadvantages. First, such methods are complicated and thus increase costs. Second, these methods do not produce layers of sufficient quality.
It would be beneficial to be able to easily produce a thin film from a first substrate on a second substrate, wherein the thin film has a thickness in the range of about 50 nanometers (nm) to about 500 nm thick, for example, and wherein the two substrates have coefficients of thermal expansion that differ by about 10% or more. Such a method could be used to produce structures such as silicon-on-quartz, silicon-on-glass, or silicon-on-sapphire, germanium-on-silicon, germanium-on-glass, or silicon carbide (SiC)-on-quartz or silicon carbide-on-glass type assemblies. The present invention now provides the skilled artisan with the ability to produce these materials.